The present invention is generally concerned with x-ray or radiographic imaging systems and methods, and is particularly concerned with a method of fabricating a high performance X-ray focal plane array (XFPA) detector for integration in such a system, as well as an imaging system and method employing the detector.
An ongoing goal of medical and other diagnostic sciences is the development of a low-cost, high quality, high resolution, real-time digital imaging system for x-rays transmitted through an opaque target, such as the human body. Such a system has the capability of providing on-line, non-invasive, multi-organ radiographic imaging. Such real-time digitized imaging eliminates the need for film, and the time required to develop such film. Unfortunately, directly digitized detection of an X-ray image is a problem because silicon used in the pixels of visible focal plane digitizing arrays or visual matrix detectors, such as charge coupled devices (CCDs), has low responsitivity and is also damaged by X-rays.
It is known to place a fluorescent or phosphorescent medium between the X-ray source and a visual matrix detector to convert the X-rays to visible light. There are still problems in using a screen of such material in front of the visual matrix detector. For example, if the screen is too thin, not enough of the X-rays will be absorbed and some will reach and damage the silicon in the matrix detector. If the screen is too thick, the induced fluorescence visible light is radiated in all directions and is also scattered, enlarging the area of the illuminated point source on the detector, thus blurring the picture and reducing the spatial image resolution. In some cases, scattered light may escape without reaching the matrix detector at all. The fluorescent or phosphorescent material may also have non-uniform properties, degrading the image quality and resolution. Some phosphorescent materials exhibit "after-glow", in other words they may continue to emit light even after the radiation source is no longer present. This may further degrade the image quality.
U.S. Pat. No. 5,519,227 of Karellas describes a structured scintillation screen which overcomes some of these problems. Regions of a transparent or semi-transparent scintillating substance are ablated to form an array of individual pixels. Each pixel is surrounded with an optically inactive material having a lower refractive index, so that the pixel is made to function as an optical waveguide. This confines the x-ray induced phosphorescence to the individual pixels and channels it to the detector. This increases resolution and detection efficiency. The method of fabrication is as follows: The substrate of phosphorescent or optically active material is exposed to electromagnetic radiation, such as a laser beam, so as to ablate the substrate in exposed regions to produce a one or two dimensional array of pixels. A mask may be placed in contact with the substrate so that the desired regions are ablated by the laser beam. Following laser processing to form the pixels, the pixels are surrounded by an optically inactive interstitial material so as to avoid optical leakage from each pixel. The pixel structure is attached via a substrate to a detector such as a CCD camera.
Other XFPA medical imagers have also been proposed, and have been introduced commercially in recent years, particularly for dental examinations. However, these imagers have, up to now, been very expensive and demonstrate marginal performance, due to the significant challenges in developing of a high performance, two dimensional XFPA detection matrix. One of the problems is that in order to replace high-resolution film radiography, the pixelated detector must have high uniformity and almost zero defects, with a resolution approaching 20 lp/mm, for good performance. All current commercial XFPA systems have demonstrated inferior imaging quality as compared with state-of-the-art commercial X-ray films, due to lack of sufficient resolution and low signal/noise.